Ludmila Carone

Context. Spectroscopic phase curves of tidally locked giant exoplanets provide us with a unique global picture of their atmospheric chemistry, as different regions of the planet rotate in and out of view. However, the spatially changing composition of tidally locked exoplanets – governed by stellar irradiation and atmospheric circulation – make phase-resolved observations exceptionally hard to interpret. Aims. Our goal is to investigate the chemistry of the hot Jupiter WASP-43 b in detail using theoretical models, considering the constraints of the James Webb Space Telescope’s MIRI phase curve. Methods. With a suite of pseudo-two-dimensional and three-dimensional photochemical models, we simulated the composition of WASP-43 b in various configurations (different metallicities and wind speeds), and we compared them with atmospheric retrieval models. Additionally, we probed the robustness of photochemical predictions by comparing the results of four one-dimensional models that have equivalent setups. Results. We confirm that disequilibrium chemistry in our theoretical models reduces the methane concentration on the planet’s night side for wind jet speeds ≳500 m s ⁻¹ . Varying the metallicity in the models induces large changes in the CO ₂ and SO ₂ concentrations, with SO ₂ producing mid-infrared absorption features in synthetic emission spectra of the night side at atmospheric metallicities ≳10× solar. Our models provide evidence for pole-to-equator circulation enhancing the CH ₄ , NH ₃ , and HCN abundances, which is nonetheless insufficient for detectable spectral features. Finally, we show that H ₂ O, CO, and CO ₂ are robustly modelled, but species affected by photochemistry (CH ₄ , NH ₃ , SO ₂ , and HCN) are more sensitive to model-specific assumptions and pathways. Conclusions. We conclude that horizontal quenching is the prime mechanism that explains the non-detection of methane in the MIRI phase curve of WASP-43 b. This mechanism requires only moderate wind speeds and is operative at various thermal structures and atmospheric metallicities. Furthermore, coupled carbon–sulfur chemistry leads to an additional decrease in methane compared to previous models in the literature that did not contain sulfur chemistry. We do not favour a high metallicity as it would have led to observable SO ₂ features in the MIRI spectra. Our study shows that phase-dependent photochemistry models are essential tools in the interpretation of hot-Jupiter phase curves, but benchmarking is needed to improve the accuracy of photochemical models in the future.

The atmospheres of sub-Neptunes provide a window into their internal structure and history, shedding light on the origin of this common, but enigmatic, class of exoplanets. However, the physical and chemical processes that shape sub-Neptunes’ transmission spectra, in particular cloud and haze formation, are not well understood. To identify possible correlations between transmission spectra and UV irradiation, the Sub-Neptune Planetary Atmosphere Characterization Experiment (SPACE) Program observed an array of sub-Neptunes and their host stars using the Hubble Space Telescope (HST), measuring the planets’ transmission spectra between 1.1 and 1.7 μ m with the Wide Field Camera 3 (WFC3) and the stars’ UV spectra with the Space Telescope Imaging Spectrograph (STIS). Here, we present the observations of HD 191939 b carried out as part of the SPACE Program, which reveal no significant spectral features in the transmission spectrum. The data deliver moderate evidence at significance levels between 2.0 σ and 3.2 σ against a cloud-free atmosphere with solar metallicity, rendering this scenario unlikely, but still possible. A supersolar metallicity of HD 191939 b might be consistent with the known trend of increasing atmospheric metallicity with decreasing planet mass. Both hydrocarbon haze formation and cloud condensation can be efficient at HD 191939 b’s zero-albedo equilibrium temperature of (880 ± 20) K, particularly in atmospheres with supersolar metallicity, possibly additionally muting absorption features.

Warm giant exoplanets exhibit strong three-dimensional temperature contrasts that can significantly alter atmospheric chemistry through quenching and photochemistry, yet transmission spectra are commonly interpreted using one-dimensional, limb-averaged models. Such simplifications may bias inferred atmospheric properties, particularly metallicity and C/O ratio. In this work, we investigate the relative influence of atmospheric composition and three-dimensional thermal structure on atmospheric chemistry and transmission spectra using WASP-69 b as a test case. WASP-69 b is a ∼900 K warm Saturn, residing in a thermal regime especially sensitive to disequilibrium chemistry. We use three-dimensional general-circulation-model-derived pressure–temperature profiles as inputs for a one-dimensional photochemical–kinetics model to resolve longitudinal and latitudinal chemical asymmetries across the atmosphere. We find that CH ₄ exhibits strong latitudinal variations linked to deep quench temperatures, while SO ₂ shows longitudinal asymmetries driven by upper-atmospheric photochemistry and irradiation geometry. In contrast, CO ₂ remains comparatively insensitive to spatial thermal variations and emerges as a robust tracer of atmospheric metallicity. Synthetic transmission spectra reveal that three-dimensional chemical asymmetries can produce spectral variations comparable to those induced by metallicity itself, demonstrating that limb-averaged interpretations can mask substantial spatial structure in warm giant exoplanet atmospheres.

As the only known habitable (and inhabited) planet in the Universe, Earth informs our search for life elsewhere. Future telescopes like the Habitable Worlds Observatory (HWO) will soon look for life on rocky worlds around Sun-like stars, so it is critical that we understand how to distinguish habitable planets from inhabited planets. However, it remains unknown if life is necessary to maintain a habitable planet, or how all of the components of an evolving planet impact habitability over time. As a first step toward answering these questions, we present a coupled interior-atmosphere evolution model of the Earth without life from 50 Myr to 5 Gyr that reproduces 19 key observations of the preindustrial Earth after 4.5 Gyr within estimated measurement uncertainties. We also produce a reflected-light spectrum covering the possible wavelength range of HWO. Our findings suggest that life may not be required to maintain long-term habitable surface conditions. The model presented here is apt for predicting the long-term habitability of Earth-like exoplanets by coupling the interior and surface evolution. By generating realistic reflected-light spectra from evolved atmospheric states, this model represents significant progress towards characterizing the observability of whole-planet evolution, which may ultimately provide a robust abiotic baseline for interpreting biosignature observations with HWO.

This study investigates the thermal stability and absorption of metal oxide clusters in exoplanetary atmospheres. Utilizing our thermochemical data, we analyze eight distinct cluster families: magnesium oxide (MgO), silicon monoxide (SiO), titanium monoxide (TiO), vanadium monoxide (VO), titanium dioxide (TiO ₂ ), vanadium dioxide (VO ₂ ), aluminum oxide (Al ₂ O ₃ ), and vanadium pentoxide (V ₂ O ₅ ). Equilibrium cluster populations as a function of gas temperature and pressure reveal distinct stability regimes. Under solar elemental abundances, (TiO ₂ ) _N and (Al ₂ O ₃ ) _N are favored at higher temperatures, while (MgO) _N and (SiO) _N dominate at lower temperatures. The computed absorption spectra exhibit strong size- and composition-dependent absorption features in the mid-infrared (8–50 μ m), many of which fall within the wavelength range accessible to JWST/MIRI . We further couple cluster thermodynamics with 3D general circulation model (GCM) outputs to investigate the cluster stability across the ultrahot Jupiters (UHJs) WASP-121 b and WASP-18 b, the hot Jupiter (HJ) WASP-39 b, and the warm Jupiter (WJ) WASP-69 b. In WASP-121 b and WASP-18 b, extreme dayside temperatures suppress large-cluster stability, yielding atmospheres dominated by metal ions at low pressures and neutral metals at depth, with limited cluster survival on the nightside and morning terminator. In WASP-39 b, larger clusters are not thermochemically favored, despite the enhanced metallicity; instead, equilibrium chemistry stabilizes smaller species, with only TiO showing a tendency toward stable larger-cluster forms, likely due to its open d-orbitals. In contrast, WASP-69 b favors the formation of larger metal oxide clusters across an extended pressure range, highlighting WJs as a favorable environment for metal oxide cluster stability.

Context . Space missions such as CHEOPS, JWST, and PLATO facilitate detailed characterization of exoplanets, particularly close-in gas giants and their wide range of global temperatures and climate regimes. Aims . The aim of this work is to provide a framework to characterize cloud and climate properties of close-in gas giants via transit depth asymmetries from the optical to the infrared ( λ = 0.33 … 10 μm). Methods . The AFGKM ExoRad 3D GCM grid provides 3D gas temperature profiles, assuming a chemical equilibrium gas-phase, for an ensemble of 50 tidally locked gaseous planets orbiting A-, F-, G-, K-, and M-type host stars. It is combined with a kinetic cloud formation model (with nucleation, surface growth and evaporation, gravitational settling, mixing, and element conservation). The end result is a set of synthetic transit spectra and evening-to-morning transit asymmetries that span all relevant climate regimes: warm ( T _global = 800 K … 1000 K), intermediately hot ( T _global = 1200 K … 2000 K), and ultrahot ( T _global = 2200 K … 2600 K). Results . WASP-39b observations suggest that clouds are iron-free and that cloud condensation nuclei are less abundant than predicted. The ensemble study shows that clouds increase transit limb differences due to asymmetries in cloud coverage and by enhancing horizontal differences in the gas temperatures. For planets in the intermediately hot climate regime, evening-to-morning differences of up to 150 ppm are suggested in the optical band, whereas differences of up to 100 ppm are implied in the infrared (2-8 μm). For ultrahot Jupiters, the observed evening-to-morning transit differences are dominated by the morning cloud for a cloud-free evening limb. The differences are strongly negative in the PLATO band (0.5-1 μm, −500 ppm), moderately negative in the near-infrared (11.5 μm, −200 ppm), and moderately positive (+100 ppm) for λ> 2 μm. For a partly cloudy evening terminator, the evening-to-morning transit asymmetry is moderately positive in the whole wavelength range. Warm Jupiter planets exhibit negligible transit asymmetries. The reported transit asymmetries are conservative. Planets 30% larger than 1 R _Jup increase the transit asymmetry signal by a factor of two. Conclusions . PLATO may help characterize climate regimes as well as cloud properties. The combination of precise PLATO and JWST transit asymmetry observations between 1-2 μm is optimal to characterize cloudy planetary atmospheres around K-A stars. JWST/MIRI observations are the most effective for planets around M stars with transit differences ≥+500 ppm for 8-10 μm.

Context. Early studies of ionization in hot Jupiter atmospheres suggest that magnetic coupling may affect their dynamics, and hence their weather and climate states. These effects may be most pronounced in ultrahot gas giants, assuming they generate their own global magnetic field. WASP-18 b, one of the best studied ultrahot Jupiters, hosts a highly ionized dayside atmosphere extending deep enough to be strongly influenced by magnetic forces. Phase curve observations suggest an effective magnetic drag, yet its impact on the atmospheric circulation remains poorly constrained. Aims. The aim is to explore the effect of magnetic drag in atmospheres with an inhomogeneous ionization on the local and global dynamics to ultimately provide a pathway to constrain the planet’s magnetic field strength. Methods. An analytical parameterization for anisotropic magnetic drag, including both Pedersen and Hall drag components, and associated frictional heating in the globally neutral atmosphere, was implemented in the 3D general circulation model ExoRad to study WASP-18 b. Fundamental plasma parameters were analyzed to explore where magnetic coupling becomes important in the atmosphere, depending on the dipolar field geometry, the ionization fraction, and the collisional coupling between charged particles and neutrals. Climate characteristics were compared for different drag formulations, to assess whether anisotropic drag physics is required to accurately capture magnetic coupling effects. Results. Anisotropic magnetic drag and frictional heating, both shaped by local ionization, strongly affect wind strength and direction in the upper atmosphere, modifying the day-night circulation and producing observable temperature asymmetries. Anisotropic drag enhances the evening-morning terminator temperature difference at 0.1 bar, and generates two off-equator hotspots with reduced eastward shift. The terminator regions are in particular susceptible to how magnetic drag is described in the model. Conclusions. Anisotropic magnetic drag damps and redirects the dayside-to-nightside winds, partially decoupling the equatorial flow at the morning terminator while maintaining the nightside jet. Locally changing drag forces and frictional heating create asymmetric temperature patterns that manifest as primary and secondary hotspot regions.

Context . With the development of ever-improving telescopes capable of observing exoplanet atmospheres in greater detail and number, there is a growing demand for enhanced three-dimensional climate models to support and help interpret observational data from space missions such as CHEOPS, TESS, JWST, PLATO, and Ariel. However, the computationally intensive and time-consuming nature of general circulation models (GCMs) poses significant challenges in simulating a wide range of exoplanetary atmospheres. Aims . The aim of this study is to determine whether machine learning (ML) algorithms can be used to predict the three-dimensional temperature and wind structure of arbitrary tidally locked gaseous exoplanets in a range of planetary parameters. Methods . We introduced a new three-dimensional GCM grid comprising 60 inflated hot Jupiters orbiting A, F, G, K, and M-type host stars, which we modelled using ExoRad . We defined four climate characteristics to characterise these planets: the dayside–nightside temperature difference, the evening–morning temperature difference, the maximum zonal wind speed, and the wind jet width. We trained a dense neural network (DNN) and an extreme gradient boosting algorithm (XGBoost) on this grid to predict local gas temperatures, as well as horizontal and vertical wind fields. To assess the reliability and quality of the ML models’ predictions, we selected WASP-121 b, HATS-42 b, NGTS-17 b, WASP-23 b, and NGTS-1 b–like planets, all of which are targets for PLATO observations, and modelled them using ExoRad and the two ML methods. For these test cases, we calculated the equilibrium gas-phase composition and transmission spectra to evaluate whether differences in local gas temperature between the general circulation model and ML approaches significantly affected the predicted chemical composition and transmission spectra. Results . With the multi-layer neural network, that is DNN, predictions for the gas temperatures are to such a degree that the calculated spectra agree within 32 ppm for all but one planet, for which only one single HCN feature reaches a 100 ppm difference. The XGBoost predictions are somewhat worse but never exceed 380 ppm differences. Generally, the resulting deviations are too small to be detectable with the observational capabilities of modern space telescopes, including JWST. For the DNN, only the WASP-121 b-like planet, which is the hottest investigated planet, shows a general offset that is smaller than 16 ppm. Horizontal wind predictions are less accurate but can capture the most general trends. As with temperature, the DNN also outperforms XGBoost in this respect. Predicting vertical wind remains challenging for all ML methods that we explored in this study. Conclusions . The developed ML emulators can, within one second, reliably predict the complete three-dimensional temperature field of an inflated warm to ultra-hot tidally locked Jupiter around A to M-type host stars. They therefore provide a fast and computationally inexpensive tool to complement and extend traditional GCM grids for exoplanet ensemble studies. The quality of the predictions is such that no, or only minimal, effects on the gas-phase chemistry and, consequently, on cloud formation and transmission spectra are expected.

As the only known habitable (and inhabited) planet in the universe, Earth informs our search for life elsewhere. Future telescopes like the Habitable Worlds Observatory (HWO) will soon look for life on rocky worlds around Sun-like stars, so it is critical that we understand how to distinguish habitable planets from inhabited planets. However, it remains unknown if life is necessary to maintain a habitable planet, or how all of the components of an evolving planet impact habitability over time. To address these open questions, we present a coupled interior-atmosphere evolution model of the Earth without life from 50 Myr to 5 Gyr that reproduces 19 key observations of the pre-industrial Earth within measurement uncertainties after 4.5 Gyr. We also produce a reflected light spectrum covering the possible wavelength range of HWO. Our findings support the view that life is not required to maintain habitable surface conditions. The model presented here is apt for predicting the long-term habitability of Earth-like exoplanets via evolving bulk properties. By generating realistic reflected light spectra from evolved atmospheric states, this model represents significant progress towards whole-planet modeling, which may ultimately provide a robust abiotic baseline for interpreting biosignature observations with HWO.

Understanding planetary habitability requires a comparative approach that explores the divergent evolutionary outcomes of Earth and Venus. The Habitable Worlds Observatory (HWO) will be uniquely positioned to conduct a statistical and physical census of terrestrial exoplanets spanning the Venus Zone (VZ) and the Habitable Zone, enabling the detection and atmospheric characterization of post-runaway greenhouse worlds (“exoVenuses”). We present an updated list of VZ exoplanets, which raises the number of known candidates to 370. We describe a science case and an observing strategy for VZ exoplanets that integrates precursor exoplanet detection data and stellar characterization with HWO direct imaging, spectroscopy across the UV/optical/IR, and spectropolarimetry. Our proposed framework emphasizes a pathway toward the diagnosis of sulfur chemistry (SO ₂ ) and aerosol physics (H ₂ SO ₄ clouds/hazes), planetary redox states (O ₂ /O ₃ false positives from hydrogen loss), and cloud microphysics detection (rainbow polarization). We quantify implications for HWO requirements, including UV access to 0.2–0.4 μ m, optical/NIR coverage to ≳1.5 μ m, inner working angle (IWA) reaching 0.3–1.5 au around nearby Sun-like stars, and the SNR/resolution needed for key features. Finally, we outline a community-driven path to producing robust demographic inferences and target selection for optimizing HWO observations.